High Precision Thickness Control on a Rolling Mill for Flat Rolled Metal

ABSTRACT

The high precision rolling design utilizes a high speed hydraulic roll position control along with a high accuracy roll gap measurement. The lower work roll position is fixed and the upper work roll is positioned by a hydraulic roll force cylinder using an inner and outer control loop. The inner loop is a cylinder position control that moves the upper work roll. The outer loop uses a measurement of the work roll gap to trim the inner cylinder positioning control. Both control loops coordinate together to provide a high precision and even strip thickness tolerance to +/−0.15 mils or less. Both sides of the upper work roll are controlled separately to achieve the overall tolerance goal.

CROSS REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO SEQUENCE LISTING, A TABLE, OR COMPUTER PROGRAM LISTING

Not applicable.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention is directed to a single stand rolling mill and associatedthickness control that is used to reduce the thickness of flat rolledmetal stock. It is particularly directed toward methods and control thatprovides high tolerance thickness control when rolling metal strip thatwill be used in stamping out coins for common use and for specializedNumismatics coinage.

(2) Description of Related Art

Some ancient coins were “cast,” meaning the metal was melted and pouredinto coin shaped molds. When the metal cooled, the mold was opened andthe newly minted coin was removed. A refinement of the process was topour fine granulated metal into the coin mold, heat the mold above themelting point of the metal, and then cool it. A more accurate weightcoin was the result.

In other coins, blanks (called flans) were often cast in molds orindividually cut from long round bars. The blanks were then struck in adie to imprint official images on the coin.

In either case, the thickness control of these methods were poor bymodern standards. Over the centuries, various methods improved the cointhickness and diameter.

In modern times, a die strikes coin blanks approximately 120 times aminute in a press to make high numbers of coins.

In more modern coin making methods, strip for coin blanks is rolled in amulti-pass rolling mill. The exit thickness of the in process stripdepends largely on the amount of space between the two work rolls. Rollposition control is provided by setting both rolls by using a mechanicalscrew and an associated indicator. The screw position sets the rollposition which in turn, sets the strip thickness. As the strip isrolled, the strip work hardens as thickness reduction is taken on thestrip. Coins have different thickness reduction amounts depending uponthe material and final desired properties.

The in process strip thickness during rolling is dependent upon a numberof factors that are difficult to control. It is difficult to grind thework rolls to a completely uniform diameter. Typical roll eccentricityis 0.5 to 1.0 mils due to grinding machine tolerances. The eccentricitywill be a permanent feature in the rolls. As the material hardens,eccentricity imprints onto the strip lessen due to an increased ‘springback’ effect in the metal.

Additionally, the work rolls will heat up during the rolling process asthe rolling process is commonly done on a ‘dry’ basis, that is, withoututilizing a rolling oil or water cooling. This heating can be uneven andcreate an additional eccentricity pattern due to varying thermalexpansion.

During initial rolling, the strip enters the roll gap and immediatelygenerates a significant striking force. The roll gap is thinner than thestrip because it is desirable to reduce the thickness for the ‘head end’of the strip. The rolling force is high enough to cause the mill housingto stretch several mils (1 mil=0.001″). The frequent strip to rollimpacts eventually cause uneven roll wear.

A multi pass rolling mill compounds the problem of roll eccentricity.The roll eccentricity from a previous pass is imprinted at a fixeddistance between sinusoidal peaks, and remains imprinted on a longerstrip during the next pass. Three or four passes are common. Theoverlaid eccentricity imprinting becomes a significant thicknessvariance as sometimes eccentricity patterns overlap in a way thatincreases the strip thickness, and sometimes decreases the stripthickness. Also, each work roll has its own eccentricity pattern thatimprints on to the strip.

It should be kept in mind that two rolls are used in the rollingprocess, and the eccentricity of each roll is independent of the other.

The US mint guarantees that certain coins (such a bullion coins) willalways be a certain minimum weight, this provides incentive to improvethe thickness control. The blanking process creates a highly accuratediameter and the coin to coin weight variance is largely dependent uponthe metal thickness of the blank.

Consequently, there is an ongoing desire to control the metal cost ofproducing coins by achieving only the desired thickness and avoiding anyexcess thickness. It is especially important in the precious metalcoinage (i.e. silver and gold). For example, the US Mint guarantees thatthe weight, content and purity of the following common bullion coins:

TABLE 1 American Eagle American Eagle Gold coin Silver coin Diameter32.70 mm 40.6 mm Thickness  2.87 mm 2.98 mm Gross weight 1.0909 troy oz1.00 troy oz (33.930 g) (31.103 g) Face value $50 $1

This provides high confidence to investors to purchase them, knowing thecoins contain the stated amount of precious metal. Sales statistics for2013 are in Table 2.

TABLE 2 2013 American Eagle Bullion Sales Coin type Pieces sold Ounces 1ounce Gold 758,500 758,500 ½ ounce Gold 57,000 28,500 ¼ ounce Gold114,500 28,500 1/10 ounce Gold 555,000 55,500 1 ounce Silver 42,675,00042,675,000

Annual sales in 2013 of all denominations of the American Eagle coin (1oz, ½ oz, ¼ oz and 1/10 oz) was 856,500 ounces. For example, if thecurrent excessive thickness is 0.25%, it would mean that the US Mint‘gives away’ 2,141 ounces. At $1,200 per ounce for a gold coin, itequates to over $2,400,000 per year. Similarly, at $50/ounce, the excesssilver for the one ounce silver eagle would be over $5,300,000.

It is desirable to achieve an improved metal thickness accuracy andprecision, to a value of +/−0.15 mils (0.0038 mm) or +/−0.13% of thedesired thickness for a one ounce American Eagle gold coin.

An additional complication is that it is very difficult to accuratelymeasure the thickness of precious metals to the desired accuracy of0.0038 mm in a dynamic rolling process. Physical contact probes, x-raybased sensors, and other methods are not practical or accurate enoughfor control purposes.

Additionally, measuring the thickness after the strip has left the rollbite is not a satisfactory method of controlling work roll imperfectionsto a fine degree. Once the strip has left the roll bite, there is noability to take corrective action on thickness variances due toimperfections in the roll bite caused by factors previously mentioned.

Additionally, in process techniques of measuring thickness variances,such as changes in rolling force, are not accurate and stable enough forthe desired high precision control. Further, these types of controlmethods are overly complicated and expensive in this field.

The strip thickness variance is currently determined by stamping outcoin blanks and weighing individual coins in a sample lot. The coin tocoin weight variance gives a correct thickness accuracy. Such thicknessdetermining methods are clearly unsuitable for dynamic control in arolling mill.

Consequently, it is highly desirable to improve the rolling thicknessvariance by at least half of the current amount. The ability to roll aprecious metal strip without concern for dynamic changes in the roll gapdue to roll wear, grinding imperfections, and thermal variances isimportant. Also there must be a satisfactory method of tightlycontrolling the strip thickness without the ability to directly measurethe thickness of the strip during rolling process.

BRIEF SUMMARY OF THE INVENTION

The high precision rolling design utilizes a high speed hydraulic rollposition control along with a highly accurate and precise roll gapmeasurement. The lower work roll position is fixed. The upper work rollis positioned by a hydraulic roll force cylinder using an inner andouter control loop. The inner loop is a cylinder position control thatmoves the upper work roll. The outer loop uses a measured roll gap totrim the inner cylinder positioning control. Both control loopscoordinate together to provide a high precision and even flat metalproduct thickness tolerance to +/−0.15 mils or less. Both ends of theupper work roll are controlled separately to achieve the overalltolerance goal.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 shows a schematic of the automatic thickness control.

FIG. 2 shows an example side view of a typical two roll stack milldesign.

FIG. 3 show a simplified layout of the light sensors that measure theroll gap in two places on either side of the strip being rolled.

FIG. 4 shows a schematic of a supporting thickness control hydraulicsystem.

FIG. 5 shows a schematic of a simplified automatic control system.

DETAILED DESCRIPTION OF THE INVENTION

It has become understood that use of a slow response electro-mechanicalscrew method in positioning the upper work roll provides an inadequatecorrection speed when attempting to control the gap between the workrolls to a high tolerance. In particular, work roll eccentricity issuesare impractical to control if the response of the roll positioningsystem does not match changes in the roll gap. It has been found byexperimentation that measuring the roll gap and controlling it with asystem that has a response of 20 milliseconds for a 0.001″ roll positionchange is adequate to achieve the desired thickness control. Thisresponse works well for a rolling speed of the strip at approximately 50fpm and work rolls approximately 8″ in diameter with a 10″ roll face.

The current inventive design was discovered after other methods ofattempting fine thickness control were unsuccessful. Attempts to controla fixed roll gap opening by setting the position of a roll forcecylinder was unable to obtain the desired thickness tolerance. Millstretch and roll eccentricity were particularly difficult compensate toachieve a very small variance in the roll gap. Also the strip beingrolled increased in hardness during the three to four rolling passes,which added an additional complexity by stretching the mill standdifferently in each pass. Use of a constant rolling force control, didnot provide the needed fine tolerance as the control did not compensatewell enough.

The control of the roll gap position must be maintained across the rollface, so the roll gap correction must be applied to both ends of thework rolls through the roll chocks. It is important that the roll gap iseven across the width of the roll during the rolling process. Otherwise,the thickness will be uneven across the strip width.

FIG. 1 shows a schematic of the automatic thickness control system withan inner roll force cylinder position control loop and outer roll gapposition control loop. A pair of work rolls 100 a,b are used to roll ametal piece that begins as approximately 3.5″×0.19″×6 feet long. A lightemitter 101 shines a uniform collimated beam to a high resolution lightreceiver sensor 102 through the work roll gap opening to determine thegap on either side of the metal strip. The roll gap sensor is able tomeasure at an accuracy of +/−0.08 mils. A thickness signal 103 is sentto a mathematic calculation 105 to compare against a roll gap setpoint104. The resulting calculation provides a roll gap error signal 111 thatis fed into a trim PID controller 107. The trim is applied to the rollforce cylinder position and will adjust the cylinder position outputfrom the trip PID controller. A roll force cylinder position PIDcontroller 109 receives feedback from a cylinder position indicator 108.The PID output then controls a hydraulic control valve 110, preferably aservovalve, which positions the roll force cylinder 112 which in turn,positions the work roll chock/bearing to move the roll on one side. Asecond roll force cylinder 113 is on the drive side of the work roll andis used to position the drive side work roll chock/bearing.

FIG. 2 shows an example side view of a typical two roll stack singlestand mill design where the upper work roll is movable for metalthickness control. A roll force cylinder 201 and associated cylinderposition sensor 202 are located within a mill housing 203. An upper workroll bearing chock 204 is attached to the roll force cylinder 201. Alower work bearing chock 205 is fixed against the mill housing 203through a roll removal slide 206 and base 207. A runout table 208provides support for the metal strip during rolling.

In an alternate embodiment, the bottom work roll is movable and theupper work roll is fixed by a stop when a strip is being rolled. In thiscase, the roll force cylinder is located under the bottom work rollchocks. The control design is the same with inner and outer controlloops.

FIG. 3 show a preferred layout of the roll gap sensors that measure bothsides of the roll gap. Two light beam emitters 303 a,b are directedbetween the upper work roll 301 and the lower work roll 302 gap to tworeceivers 304 a,b. The light emitters and receivers are on either sideof the strip 305 being rolled. To simplify the figure, the two workrolls are shown without bearings or bearing chocks.

In an alternate embodiment, only a single light beam is used for gapfeedback. In this alternate embodiment, the gap measurement is used tocontrol the position of both roll force cylinders. One design methodused to ‘level the mill,’ i.e. setting the roll gap equally across thewidth, is done by applying an even rolling force across the work rollface and noting any difference in roll force cylinder position betweenthe two sides of the work roll. This offset is manually used in thecylinder setpoint.

The roll gap sensors have a measuring accuracy of +/−0.08 mils. Thesensor captures 16,000 samples per second and a maximum 30 mm gap can bemeasured. The sensors are preferably mounted on each side of the roll,as close to the edge of the roll face as possible. They are positionedso as to be protected from errant strip tracking during the rollingprocess. The sensor mounting must also be stable, and free of vibration.

It was found that when the roll gap sensor is used in an inner/outerloop control as described, the desired tight rolling thickness accuracywas achieved to an acceptable, commercial level when 95% of the striplength was within the desired thickness range. When a roll gap sensorwas not used, roll position alone did not provide the desired thicknesstolerance. The hydraulic control valve and hydraulic system weredesigned to move the roll force cylinder quickly, and provide for a 20millisecond response when making a 0.001″ roll gap correction. Theservovalve design flow rate was 2.5 gpm. This high response designprovides for gap corrections at up to 125 times per roll rotation whenrolling at 50 fpm.

FIG. 4 shows a high pressure hydraulic system that provides the neededpressure and flow rate to support the hydraulic servovalve. A pump 401is mounted near a tank 408 and supplies pressurized hydraulic fluid to afilter 402. The pressurized fluid line is connected to a dump valve 403which is used to drop the pressure in the hydraulic system. The dumpvalve is useful for rolling emergencies, startup sequence, and whendoing maintenance on the mill stand to ensure that hydraulic pressure isshut off. On the pressurized line a servovalve enable valve 404 is usedto turn off the pressure to the servovalve for safety and operationalreasons. An accumulator 405 provides immediate fluid for the highresponse servovalve 406 which is connected to the thickness controlsystem as mentioned in FIG. 1. A roll force cylinder 407 is used todevelop the needed rolling force in the mill stand. A connection 409indicates that there is a replicate hydraulic control system (items404-407) for the second roll force cylinder.

Not shown in FIG. 4 are various other operational and maintenance valvesfor roll changing and small flow restricting valves to smooth theoperation of the illustrated hydraulic valves.

FIG. 5 shows a design schematic of a simplified automatic control systemwith two substantially replicated control loops. The controls are asimplified version of FIG. 1. A work roll gap measuring sensor outputs asignal 501 which represents the roll gap on one side. That signal inputsto a work roll gap PID controller 502. The operator inputs a roll gapsetpoint 503 to set the exit thickness of the rolling pass of a metalstrip. The setpoint will vary with each rolling pass. The PID controller502 controls a hydraulic control valve 504, which is preferably aservovalve. The hydraulic control valve then moves the roll forcecylinder 505 which in turn positions the upper work roll. As shown, aparallel, replicated control loop moves the second sides of the upperwork roll in order to position it. Not shown are needed hydraulic andcontrol components which will move the work rolls for maintenance,various operational reasons, etc. unrelated to thickness control.

Also illustrated are components of the system 506 that drives the workroll rotation which includes a motor, gearbox with two output shafts,and two spindles which connect to the upper and lower work rolls to thegearbox.

The stock used in rolling is relatively small—approximately 3.5 incheswide×0.28 inches thick×6 feet long. In one embodiment, a 40% thicknessreduction is accomplished in 3 or 4 rolling passes. The strip increasesin length by approximately 2½ times. Other thickness reductions andnumber of rolling passes are possible. However, productivity improves ifthe number of passes is reduced by suitably increasing the thicknessreduction on each pass.

The development of hydraulic pressure in the roll force cylinders isaccomplished by a supporting high pressure hydraulic system and a highresponse servovalve. A 6 gpm hydraulic pump at 2,500 psi providessuitable pressure and hydraulic supply for both roll force cylinders.The roll force cylinders were a 160 mm bore and a 65 mm stroke. Aposition sensor is mounted on each roll force cylinder with 0.1 micronresolution. These values are not the only ones possible. The cylinderposition is measured for position control feedback. The hydraulic systemmay also be utilized for support systems such as equipment used whenchanging rolls.

The outer work roll gap control loop preferably updates at 5 millisecondintervals, and the inner hydraulic cylinder positioning loop updates at1 millisecond intervals. However, these values are not the only possiblevalues. It was found that setting the outer loop control to a 50 msupdate produced acceptable, high tolerance results. Overall, whenconsidering the fast response of the hydraulic control valve andassociated hydraulic system, it is preferable for the design to provideat least a 50 millisecond response when moving the work roll 0.001inches.

It was found that the mill design stretched about 5 mils or so when thestrip entered the roll bite. The sudden increase in force opened theroll gap and caused off thickness above an acceptable amount. Thecontrol system then quickly corrects by increasing the rolling force,and the desired thickness quickly settles out. Approximate 4″ of thebeginning strip ‘head end’ as measured on the final rolled length, wasfound to be at an undesirable tolerance and will be cut off beforeproceeding to create blanks for minting coins. There was no ‘tail end’loss.

Future efforts may be utilized to reduce or eliminate the small lengthof head end metal that is out of specification.

The overall rolling system design also incorporates a record keepingdata logger that is useful for recording rolling values, and for futurerecords and troubleshooting.

The operator interfaces with the rolling mill through work roll gapsetpoints and rolling force cylinder position setpoints on both sides ofthe mill. The roll force cylinder setpoint is set to an estimated valuewhere the cylinder position will be when rolling. The operator sets theroll gap based on the desired exit strip thickness. The roll forcecylinder position setpoint is primarily used for maintenance functions,such as roll change, and also as a back-up thickness control system whenthe roll gap sensor fails. When rolling, the outer loop design will, ineffect, take over the control of roll force cylinder position. The innerloop roll force cylinder position setpoint becomes a starting referenceand the outer control loop will quickly take over the cylinder position.

The rolling design is based on a dry method, that is, no rolling oils orcooling water is used. This is preferable for the finished product, andalso for overall mill stand cleanliness that will keep the light sensorsused to measure the rolling gap clean. Additionally, if fluid is allowedon the roll body surfaces, the variances in fluid thickness will causeinaccurate gap measurement.

It was not found necessary to utilize work roll bearings with a tightrolling tolerance. The ability to directly measure the work roll gapcompensates for any bearing issues.

The overall rolling control system is designed to level the mill (i.e.create the same exit thickness across the strip width) as part of thethickness control system. It is also unnecessary to include a millstretch calculation in the thickness control system. The effects of millstretch are taken care of by the direct roll gap measurement with thelight sensors.

Both work rolls are driven by a single A/C motor through a gearing boxwith two exit shafts. The exit shafts are in turn are connected to thetwo work rolls through spindles. The spindles allow for roll changingand also for the upper work roll to move up and down. The work rolls arepreferably chrome plated to reduce wear, and preferably have a smoothfinish.

If a roll gap sensor fails, or if the operator needs to intervene due toa control issue, the operator can switch to direct roll force cylinderposition control. This allows the operator to complete a pass eventhough the final tolerance will not be as tight. In this case, abumpless transfer between direct roll gap control and cylinder positioncontrol is utilized. This provides a ‘fail safe’ control design.

Use of the term ‘strip’ should not be restrictive as to the potentialmaterial dimensions that is rolled by the teachings of this invention.Flat rolled metal product such as bar, plate, and sheet dimensions areequally rolled to a high precision tolerance.

While various embodiments of the present invention have been described,the invention may be modified and adapted to various operational methodsto those skilled in the art. Therefore, this invention is not limited tothe description and figure shown herein, and includes all suchembodiments, changes, and modifications that are encompassed by thescope of the claims.

We claim:
 1. A single stand rolling mill with a thickness control systemdesigned to roll a flat metal product comprising: A) a pair of workrolls, wherein one work roll is designed to be separately movable oneach side of said rolling mill for metal thickness control purposes, B)wherein said thickness control system is designed to position saidmovable work roll on both sides of said rolling mill, C) wherein saidthickness control system comprises the following features on each sideof said movable work roll: a) a roll force cylinder designed to positionsaid movable work roll, b) a roll gap sensor, wherein said roll gapsensor has an accuracy of +/−0.08 mils or less, c) an outer roll gapposition control loop, wherein said outer roll gap position control loopis connected to said roll gap sensor, d) wherein said outer roll gapposition control loop is additionally connected to an inner roll forcecylinder position control loop, e) wherein said an inner roll forcecylinder position control loop is connected to a hydraulic controlvalve, f) wherein said hydraulic control valve is connected to said rollforce cylinder, and g) said hydraulic control valve and associatedhydraulic system is designed to move said movable work roll 0.001 inchesin 50 milliseconds or less, D) whereby said thickness control system isdesigned to roll said flat metal product within +/−0.15 mil of a desiredthickness.
 2. The single stand rolling mill according to claim 1 whereinsaid pair of work rolls are designed to roll said flat metal product indry conditions.
 3. The single stand rolling mill according to claim 2wherein said thickness control system design requires multiple rollingpasses.
 4. The single stand rolling mill according to claim 2 whereinsaid thickness control system is designed to roll said flat metalproduct used for coins.
 5. The single stand rolling mill according toclaim 4, wherein said flat metal product comprises gold, silver, andplatinum products useful for said coins.
 6. The single stand rollingmill according to claim 1 wherein said movable work roll is an upperwork roll.
 7. The single stand rolling mill according to claim 1 whereinsaid roll gap sensor uses to measure a gap between said pair of workrolls.
 8. A single stand rolling mill with a thickness control systemdesigned to roll a flat metal product comprising: A) a pair of workrolls, wherein one work roll is designed to be separately movable oneach side of said rolling mill for metal thickness control purposes, B)wherein said thickness control system is designed to position saidmovable work roll on both sides of said rolling mill, C) a roll gapsensor, wherein said roll gap sensor has an accuracy of +/−0.08 mils orless, D) wherein said thickness control system comprises the followingfeatures on each side of said movable work roll: a) a roll forcecylinder designed to position said movable work roll, b) an outer rollgap position control loop, wherein said outer roll gap position controlloop is connected to said roll gap sensor, c) wherein said outer rollgap position control loop is additionally connected to an inner rollforce cylinder position control loop, d) wherein said an inner rollforce cylinder position control loop is connected to a hydraulic controlvalve, e) wherein said hydraulic control valve is connected to said rollforce cylinder, and f) said hydraulic control valve and associatedhydraulic system is designed to move said movable work roll 0.001 inchesin 50 milliseconds or less, E) whereby said thickness control system isdesigned to roll said flat metal product within +/−0.15 mil of a desiredthickness.
 9. The single stand rolling mill according to claim 8 whereinsaid pair of work rolls are designed to roll a flat metal product in dryconditions.
 10. The single stand rolling mill according to claim 9wherein said rolling mill thickness reduction design requires multiplepasses.
 11. The single stand rolling mill according to claim 9 whereinsaid thickness control system is designed to roll said flat metalproduct used for coins.
 12. The single stand rolling mill according toclaim 11, wherein said flat metal product comprises metals that aregold, silver, and platinum.
 13. The single stand rolling mill accordingto claim 8 wherein said movable work roll is an upper work roll.
 14. Asingle stand rolling mill with a thickness control system comprising: A)a pair of work rolls, wherein one work roll is designed to be separatelymovable on each side of said rolling mill for metal thickness controlpurposes, B) wherein said thickness control system comprises thefollowing features on each side of said movable work roll: a) a rollforce cylinder designed to position said movable work roll, b) a rollgap sensor, wherein said roll gap sensor has an accuracy of +/−0.08 milsor less, c) an inner roll force cylinder position control loop, d) anouter roll gap position control loop, and e) a hydraulic servovalvedesigned to move said movable work roll 0.001 inches in 50 millisecondsor less, C) wherein said thickness control system is designed to roll aflat metal product used for coins, and D) whereby said thickness controlsystem is designed to roll said flat metal product within +/−0.15 mil ofa desired thickness.
 15. A single stand rolling mill with a thicknesscontrol system comprising: A) a pair of work rolls, wherein one workroll is designed to be separately movable on each side of said rollingmill for metal thickness control purposes, B) wherein said thicknesscontrol system comprises the following features on each side of saidmovable work roll: a) a roll force cylinder designed to position saidmovable work roll, b) a roll gap sensor, wherein said roll gap sensorhas an accuracy of +/−0.08 mils or less, c) a roll gap position controlloop, and d) a hydraulic servovalve designed to move said movable workroll 0.001 inches in 50 milliseconds or less, C) wherein said thicknesscontrol system is designed to roll a flat metal product used for coins,and D) whereby said thickness control system is designed to roll saidflat metal product to within +/−0.15 mil of a desired thickness.